1. Field of the Invention:
This invention relates to a vibration isolation system and particularly to such a system wherein inertia forces resulting from vibration-induced acceleration of an inertia mass that is mounted on the vibrating body are transmitted back to the body to oppose and reduce the effects of the undesired vibration.
2. Description of the Prior Art:
A spectrum of vibratory displacement and rotational motion about three mutually perpendicular axes, namely, the pitch, roll and yaw axes of a helicopter is descriptive of the operating environment in which a helicopter rotor, and the transmission to which it is attached, must operate. Vibration induced accelerations of the rotor produce displacements and rotations about the principal axes of the helicopter which must be attenuated, preferably within the rotor-transmission assembly, to avoid the undesirable vibratory accelerations from being transmitted to the fuselage structure to which the transmission is attached. To reduce the vibration effects to the helicopter structure requires, then, isolation of six degrees of freedom.
It is well known that the helicopter structure vibrating at a substantially constant frequency through the forcing effect of a vibrating rotor-transmission assembly to which the fuselage is attached may be effectively isolated from the forced vibration spectrum imposed by the rotor by interposing a second mass, upon which the induced accelerations are applied, provided the second mass can transmit its inertia forces to the helicopter structure. In this way, the inertia forces of the second mass can be made to oppose the directional sense of the induced vibration by tuning the natural frequency of the second mass isolator system to the principal frequency of the induced vibration. To the extent that the fuselage structure receives inertia forces from the second mass that are in phase with, oppositely directed to and of equal magnitude with the rotor loads carried to the fuselage, the isolation system can dynamically uncouple the rotor and the fuselage.
However, isolation systems of this kind are capable of isolating the vibration induced effects generally about one axis of the helicopter; vibration accelerations about the remaining axes and the displacement forces directed parallel to these axes require additional isolators. For example, U.S. Pat. No. 4,088,042, issued May 8, 1978 to myself and others, and my U.S. Pat. No. 4,140,028, issued Feb. 20, 1979 disclose vibration isolation systems which include a plurality of single direction isolators.